WASHINGTON — The three congressional committees investigating Russian interference in the 2016 election often overlap but rarely coordinate, creating a legal quagmire for witnesses and challenges for special counsel Robert Mueller as he conducts a separate criminal probe.

The committees have been tripping over one another in recent months in the competition for witnesses and information.

One of the most egregious examples — according to four congressional aides who requested anonymity because they weren't authorized to speak publicly about an ongoing investigation — occurred in July when investigators for the House Intelligence Committee traveled to London to try to contact Christopher Steele, the former British spy who compiled a dossier on Donald Trump and his alleged ties to the Russian government.

Republicans on the House panel did not tell the committee's Democrats, Mueller, or the Senate Intelligence Committee that they were sending two staff investigators to London, according to congressional aides.

The House staffers didn't end up talking to Steele, but other investigators fear they may have spooked the ex-spy, who has been lying low since the racy 35-page dossier he compiled on Trump became public in January when it was published by BuzzFeed. Steele compiled the dossier for Fusion GPS, a U.S. firm that did opposition research on Trump for both Republicans and Democrats who opposed him.

The Senate Intelligence Committee has been trying to negotiate its own interview with Steele, whose dossier contains unverified accusations of misconduct by Trump and collusion between Trump and the Russian government. The president has repeatedly denied any collusion by himself or his campaign during the 2016 election.

"A hallmark of congressional committees is that they are all little fiefdoms," said Andrew Wright, an associate professor at Savannah Law School in Georgia, former staff director of the House Oversight and Government Reform Committee, and former associate counsel to ex-president Barack Obama. "Unless (House and Senate) leadership takes a heavy hand, the committees are not going to coordinate much."

Each investigating committee — the House Intelligence Committee, the Senate Intelligence Committee and the Senate Judiciary Committee — has its own unique personality and culture and its own way of approaching the Russia investigation.

The House panel has been plagued by internal divisions, with Chairman Devin Nunes, R-Calif., ostensibly stepping aside from the Russia investigation but continuing to issue subpoenas and write letters to administration officials about the probe.

Nunes announced in April that he would step aide from the Russia probe and allow Rep. Mike Conaway, R-Texas, to take charge while the House Ethics Committee looks into allegations that Nunes may have mishandled classified information.

Yet Nunes wrote a Sept. 1 letter to Attorney General Jeff Sessions threatening to hold him and FBI Director Christopher Wray in contempt of Congress unless they hand over documents about the dossier Steele compiled.

"It sounds like there's two investigations going on in House Intel — one by the committee and one by the recused chairman," Wright said.

Rep. Devin Nunes leaves a classified briefing at the U.S. Capitol on April 27, 2017.(Photo: Jim Lo Scalzo, European Pressphoto Agency)

In contrast, the Senate Intelligence Committee, led by Chairman Richard Burr, R-N.C., and Vice Chairman Mark Warner, D-Va., has come to be viewed as conducting the most bipartisan and serious inquiry. Burr and Warner often release joint statements and talk to reporters together. They frequently speak favorably of each other and have avoided partisan sniping, at least publicly.

"I think (Burr) is trying to run a stand-up investigation," Wright said.

The Senate Judiciary Committee appears to be somewhere in between, with Chairman Chuck Grassley, R-Iowa, and California Sen. Dianne Feinstein, the panel's senior Democrat, sometimes appearing to be on the same page and sometimes not.

A recent report by CNN revealed that the Department of Justice refused to allow two top FBI officials to talk to the Senate Judiciary Committee about Trump's firing of former FBI director James Comey in May. Comey was fired in the midst of the agency's investigation of possible collusion between the Trump campaign and Russian officials. The Justice Department cited Mueller's investigation as the reason for its refusal.

The Judiciary Committee, in turn, is not giving Mueller unfettered access to the transcript of its recent closed-door interview with Trump Jr., according to CNN.

However, legal experts say Mueller won't have to fear the most damaging action by Congress — a grant of immunity to witnesses that Mueller may want to prosecute.

Former White House national security adviser Michael Flynn has sought immunity from the Senate Intelligence Committee in exchange for his testimony, but the panel has denied his request.

Congress has been reluctant to grant immunity to witnesses since the late 1980s, when congressional investigators gave it to key figures in the Iran-Contra arms-for-hostages scandal, thwarting prosecutors' efforts to send them to prison.

"Since then, no committees want to touch immunity with a 10-foot pole," said Charles Tiefer, a professor at the University of Baltimore School of Law and the special deputy chief counsel for the House Iran-Contra Committee's investigation during the Reagan era.

Even if committee chairmen were willing to offer immunity, it now takes a two-thirds vote of the full committee to approve such a deal, making it all but impossible in a divided Congress, Tiefer said.

Not only are targets of the Russia investigation unlikely to get immunity from Congress, they have to worry that any discrepancies in their testimony before the three panels could result in perjury charges against them, said William Treanor, dean of the Georgetown University Law Center and associate counsel in the Office of Independent Counsel during the Iran-Contra investigation.

"If you're a lawyer advising one of these witnesses, you're going to be stressing the importance of consistency," Treanor said. "If witnesses say something different to each committee, they are setting themselves up for a possible perjury charge."

Former FBI director Robert Mueller, the special counsel probing Russian interference in the 2016 election, departs Capitol Hill following a closed-door meeting on June 21, 2017.(Photo: Andrew Harnik, AP)

With three committees going their own way in the Russia probe, it's possible they could reach three different conclusions about Russia's interference in last year's election and whether the Trump campaign colluded with Russian officials.

"If the committees come out with different conclusions, I think that undermines their credibility," Treanor said. He said the public would likely rely on Mueller's investigation as the most reliable one under that scenario.

"I think most people think of the special prosecutor as above the fray as a lawyer, not a politician," the dean said.

However, it's not unusual for congressional committees to come out with conflicting reports or to have dissenting views attached to the main report, Tiefer said.

"The report that's the most sexy will be the one that gets public attention," he said.